Method of timing laser beam pulses to regulate extreme ultraviolet light dosing

Abstract

Described herein are embodiments of a method to control energy dose output from a laser-produced plasma extreme ultraviolet light system by adjusting timing of fired laser beam pulses. During stroboscopic firing, pulses are timed to lase droplets until a dose target of EUV has been achieved. Once accumulated EUV reaches the dose target, pulses are timed so as to not lase droplets during the remainder of the packet, and thereby prevent additional EUV light generation during those portions of the packet. In a continuous burst mode, pulses are timed to irradiate droplets until accumulated burst error meets or exceeds a threshold burst error. If accumulated burst error meets or exceeds the threshold burst error, a next pulse is timed to not irradiate a next droplet. Thus, the embodiments described herein manipulate pulse timing to obtain a constant desired dose target that can more precisely match downstream dosing requirements.

What is claimed is: 
     
       1. A method of regulating a dose of energy produced during continuous burst mode of an EUV light source comprising:
 (a) beginning a burst having a predetermined energy dose target; 
 (b) timing by the laser controller a trigger to pulse a laser beam to irradiate a droplet during the burst; 
 (c) sensing EUV energy generated by the droplet; 
 (d) calculating by the laser controller a current dose error for the droplet based on the sensed EUV energy and the energy dose target; 
 (e) accumulating by the laser controller a burst error based on the current dose error and a running burst error calculated for one or more preceding droplet during the burst; 
 (f) repeating steps (b)-(e) for a next droplet when the burst is not finished and the accumulated burst error does not meet or exceed a threshold burst error; 
 (g) mistiming by the laser controller the trigger to pulse the laser beam to not irradiate the next droplet when the burst is not finished and the accumulated burst error meets or exceeds the threshold burst error; and 
 (h) repeating steps (c)-(g) until the burst is finished. 
 
     
     
       2. The method of  claim 1  wherein the current dose error equals the sensed EUV energy minus the energy dose target. 
     
     
       3. The method of  claim 2  wherein the accumulated burst error equals
   running burst error+(gain*dose error). 
 
     
     
       4. The method of  claim 3  wherein the gain is 1. 
     
     
       5. The method of  claim 1  wherein the current dose error equals the energy dose target minus the sensed EUV energy. 
     
     
       6. The method of  claim 5  wherein the accumulated burst error equals
   running burst error+(gain*dose error). 
 
     
     
       7. The method of  claim 6  wherein the gain is −1. 
     
     
       8. A system for regulating a dose of energy produced during continuous burst firing of an EUV light source configured to generate an energy dose target comprising:
 a drive laser configured to pulse a laser beam when a trigger is received; 
 a sensor configured to sense EUV energy generated by irradiation of a droplet; and 
 a controller configured to:
 (a) time the trigger to pulse a laser beam to irradiate a droplet during the burst; 
 (b) calculate a current dose error for the droplet based on the sensed EUV energy and the energy dose target; 
 (c) accumulate a burst error based on the current dose error and a running burst error calculated for one or more preceding droplet during the burst; 
 (d) repeat steps (a)-(c) for a next droplet when the burst is not finished and the accumulated burst error does not meet or exceed a threshold burst error; 
 (e) mistime the trigger to pulse the laser beam to not irradiate the next droplet when the burst is not finished and the accumulated burst error meets or exceeds a threshold burst error; and 
 (f) repeat steps (b)-(e) until the burst is finished. 
 
 
     
     
       9. The system of  claim 8  wherein the current dose error equals the sensed EUV energy minus the energy dose target. 
     
     
       10. The system of  claim 9  wherein the accumulated burst error equals
   running burst error+(gain*dose error). 
 
     
     
       11. The system of  claim 10  wherein the gain is equal to 1. 
     
     
       12. The system of  claim 8  wherein the current dose error equals the energy dose target minus the sensed EUV energy. 
     
     
       13. The system of  claim 12  wherein the accumulated burst error equals
   running burst error+(gain*dose error). 
 
     
     
       14. The system of  claim 13  wherein the gain is −1.